The present invention relates to vehicle rearview mirrors. One problem with rearview mirrors is that they can be very annoying at night. The lights of a following car can be blinding when they reflect into the driver's eyes from the rearview mirror.
With respect to interior mirrors, a common commercial solution to this problem is to utilize a prism mirror which can be mechanically shifted from a day position wherein one receives the maximum reflection of light entering the mirror to a night position where one sees only a small fraction of the light entering the mirror.
Studies show that in daylight, the mirror must have an image brightness of about 35%. Since tinted back windows usually transmit about 70% of the incident light, the interior mirror must reflect at least 50% of the incident light. Even this is unsatisfactory in terms of consumer acceptance. It is generally felt that consumer acceptance requires that the interior mirror have a reflectance of 60-70% in the daylight mode. In contrast for the night mode, interior mirror reflectance is preferably less than about 10%..sup.1 FNT .sup.1 Reflectance as used throughout refers to the percentage of incident light reflected as measured by the Society of Automotive Engineers Test SAEJ964A.
One problem with mechanically shiftable prism mirrors is that slight adjustments of the mirror position, either accidentally or intentionally, can result in a complete change in the reflectivity which the driver sees. Following such adjustments, the driver must again shift the mechanical mechanism to change the mode of the mirror.
Another problem with such mirrors is that if they are not adjusted just right, one sees the ghosts of objects or people in the backseat in the rearview mirror at the same time one sees the lights of a following car. This can be disconcerting.
With exterior mirrors, the prismatic solution does not work. No solution to the problem is commercially available. Often, drivers simply adjust the outside rearview mirrors to an inoperative position. This is especially unsafe in the case of vehicles having no interior mirror.
It has been suggested that, in order to obviate the above problems, it would be desirable to darken a mirror electro-optically. By applying a charge to a liquid crystal rearview mirror, one could immediately darken the mirror to a night driving mode. However, the use of a liquid crystal device as a rearview mirror has a number of significant drawbacks, not the least of which is excessive cost.
A liquid crystal mirror is too dark in the daylight mode, e.g., around 50% and are too reflective in night mode, e.g., around 15%. A liquid crystal device would require two pieces of glass which, in addition to being costly, could cause the driver to see double images. Also, it is extremely difficult to make a double glass liquid crystal mirror shatterproof. Any breakage of the seal of the liquid crystal would result in material oozing out of the mirror. Further, it would be extremely difficult to make a liquid crystal mirror in a concave or convex configuration as is often desired for rearview mirrors.
If one employed some type of automatic light sensing system to control the liquid crystal mirror, one would encounter "flickering". As a photocell sensed the lights of a following car, it would apply a voltage to the liquid crystal device which would immediately trigger it to its darkened state. As the car passed, the mirror would immediately flip back to its more reflective mode.
It has been suggested that such drawbacks could be overcome by darkening a mirror electrochromically in response to night driving conditions, using solid state electrochromic materials. Electrochromism involves the use of layers of electrochromic material which darken in response to an applied voltage. Persistent electrochromic materials stay darkened in response to an initial voltage input. They then lighten again in response to a neutralizing voltaic input. The extent of darkening and lightening can be variably controlled in response to variable input from a light sensor, this eliminating any flickering problem.
Unfortunately, such electrochromic mirrors heretofore developed have been either too dark in the daylight, i.e., reflecting less than 70% of incident light, or have been too reflective at night, i.e., reflecting more than 10% of incident light. Also, such mirrors have been too slow in shifting from one mode to the other. This is extremely annoying to the driver who wants to be able to adjust his mirror instantly from daylight to night mode or from night to daylight mode.
Some electrochromic mirror suggestions have involved the use of extremely reactive reflective materials, such as alkali metals. These are extremely dangerous to work with and are difficult to protect environmentally in an automobile. Others have involved the use of very expensive materials. Some electrochromic devices also use two pieces of glass and thus suffer the same shattering and convex fabrication problems discussed above for liquid crystal mirrors.
It is probably for these reasons that to date, there are no commercially available electrochromic rearview mirrors.